A.T. Robertson discusses this on pp. 218f of his big grammar. Apparently, Ionic SS normally prevails over Attic and Boeotian TT, but "it does not seem possible to reduce the usage to an intelligent rule." The little Liddell and Scott seems to have entries under both spellings except in some cases where the TT entry would immediately follow the SS one.

Ihave a small, technical question (which I guess I'd know if I'd
learned Greek the LaSor way and notthe Machen way, by rote). I've
encountered a few verbs, in particular (HSSAOMAI, that change the
double Sigma to a double Tau, in an apparently unpredictable way
(double Tau in the present/mid.pass part., double Sigma in the Future).
Is there are sort of "rule" by which one can guess if they see a verb
form with a double TT in it that the dictionary form has a double
Sigma? It makes finding the word a challenge. Thanks.